1. Field of the Invention
The present invention relates to a sheet processing apparatus that carries out post-processing on a fold of a sheet bundle stacked and folded.
2. Description of the Related Art
Conventionally, there have been known finishers having a mechanism to collectively fold a plurality of sheets and sheet bundles stacked and folded on a stacking tray in a tile-stacking manner, as finishers that carry out various types of post-processing on sheets on which images have been formed by an image forming apparatus. In this case, roughly, when a set of sheets consisting of twenty or more sheets are collectively folded, a fold top of a finished sheet bundle is curved. Because such a sheet bundle is poorly folded, the sheet bundle opens immediately after it is folded, and it is poor-looking in appearance. Moreover, such a poorly-folded (that is, weakly-folded) sheet bundle opens on its edge side, and a front cover surface tilts. It is thus difficult to stack a number of sheet bundles on the stacking tray.
To cope with this problem, there has been proposed a method and apparatus that brings a fold top of a poorly-folded sheet bundle into pressure to thereby flatten the fold top (see U.S. Pat. No. 6,692,208). FIGS. 17A and 17B are views schematically showing an arrangement of a conventional apparatus which flattens a fold top of a poorly-folded sheet bundle, and how the apparatus operates. In this conventional apparatus, first, as shown in FIG. 17A, a fold top 706 of a sheet bundle 701 is folded between holding members 702 and 703 by a pushing plate 705. Then, as shown in FIG. 17B, the fold top 706 is brought into contact with a stop plate 704, then the pushing plate 705 is pulled out, and the sheet bundle 701 is held by the holding members 702 and 703.
However, in the case of the apparatus and method shown in FIGS. 17A and 17B, because holding by the holding members 702 and 705 is started after the pushing plate 705 brings the fold top 706 into contact with the stop plate 704, the flattening process takes a long time, and a sheet bundle to be processed next stands by for a long time. Therefore, the problem of a reduction in productivity arises.